Regulation of the number of insulin receptors on the cell surface plays a critical role in determining insulin sensitivity. In order to study the mechanism of transcriptional regulation of the insulin receptor gene, we have cloned the 5' end of the human and mouse insulin receptor gene. We have begun to characterize the proximal promoter and find it has many features of a "housekeeping gene". In addition, we have localized a weak enhancer upstream of the promoter that is conserved in both the human and mouse promoters. This enhancer sequence binds nuclear proteins from many different cell lines. The proximal promoter is probably responsible for the low level expression of the insulin receptor gene which occurs in most cell types. Recently we have identified a region of the first intron which may be involved in tissue specific regulation. This region seems to be responsible for the 10-fold induction of the insulin receptor gene during adipocyte differentiation in 3T3-L1 cells in vitro. We inquired whether this sequence in the first intron had the same function. To address this question, we constructed transgenic mice in which a reporter gene (chloramphenicol acetyl transferase) is driven by the insulin receptor promoter plus the putative regulatory element in intron 1. However, the CAT reporter gene was expressed primarily in brain. Little if any CAT activity was detected in adipose tissue. Thus, 3T3-L1 cells in vitro appeared not to be a perfect model for adipose tissue in vivo. Accordingly, we have developed a system for transfecting primary cultures of rat adipocytes. This system can be used to map cis-acting elements in the promoter using expression vectors with a luciferase reporter gene.